A fundamental challenge in neuroscience is to determine how neuronal circuits control behavior and how plasticity, or changes in behavior, is brought about by experience- dependent changes in circuitry. To address this basic challenge we must first identify synaptically-connected neurons within a functional circuit and second, we must be to visualize connected neurons in living animals under conditions in which circuits and behavior are induced to change. This will allow scientists to test whether experience- dependent changes in neuronal connections underlie changes in behavior. The goal of our proposal is to generate tools to visualize and manipulate synaptically-connected neurons within functional circuits in living animals. We propose to accomplish this goal using a unique strategy in which we identify endogenous proteins which cross synapses and use these proteins to transport the gal4 transcriptional activator into postsynaptic neurons, where it will induce amplified expression of UAS-driven reporters or genes of interest.